The role of carbon dioxide as a greenhouse gas and its contribution to global warming is widely recognised by both scientists and governmental agencies. It is now imperative that new reactions and processes are discovered that can either efficiently store or utilise the abundant and renewable CO2 resource in an environmentally friendly manner. However, this presents a fundamental challenge because carbon dioxide is kinetically and thermodynamically stable.
The storage of the non polar CO2 molecule in a solid form is difficult, but progress is being made using a range of high surface area macro and microporous materials, such as inorganic materials (e.g. alumina, silicas and zeolites), organic materials (e.g. activated carbons), as well as complex metal-organic frameworks (MOFs).[1] Arguably a more desirable outcome would be the low temperature conversion of CO2 into alternative chemicals useful for both energy production and as chemical feedstocks. Simultaneously this would have the additional benefit of reducing our requirements on fossil fuel reserves. Homogenous and heterogeneous processes have been developed that utilise CO2 to produce CO, formic acid and its derivatives.[2] However, these reactions are far from ideal, so further breakthrough technologies are required.
There is particular interest in the reduction of CO2 by H2 to give renewable sources such as methanol. Methanol is considered to be a valuable product because it can be safely stored and transported. In addition, world demand for methanol is currently increasing enormously because of its role as a precursor to many useful organic chemicals (e.g., formaldehyde, acetic acid); a substitute for traditional fossil fuels; and in the generation of electricity in fuel cells.
CO2 hydrogenation has been extensively developed using solid oxide catalysts, but was only first reported in homogeneous solution by Sasaki and co-workers using Ru3(CO)12—KI mixtures.[3] However, these systems tend to give distributions of C1 products, namely CO, CH3OH and CH4.
Furthermore, the transition metal oxide catalysts used in these hydrogenation reactions can be expensive, and their toxicity can also give rise to potential environmental and/or disposal problems.
Accordingly, there is a need for improved processes for the manufacture methanol from CO2 which can: (i) provide the methanol in a pure form (i.e. without other C1 by-products); (ii) be operated at relatively low temperatures and pressures; and (iii) avoid the use of transition metal catalysts.